This invention pertains to the preparation of acetic acid by the fermentation of an aqueous nutrient medium containing assimilable sources of carbon, nitrogen and inorganic substances and more particularly to the use of a variant of Clostridium thermoaceticum at pH's as low as 4.5.
Acetic acid is regularly made by fermentation of sugars. In the course of making wine from grapes, failure to exclude air results in formation of vinegar. The so-called "distilled vinegar" of commerce is made by microbial oxidation of ethanol from either synthetic or fermentation sources.
The vinegar process takes place in two distinct steps. The first is fermentation of the sugar to ethanol with a yeast in yields of about 45%. The second step is a microbial oxidation of the alcohol in the vinegar stock to acetic acid with the organism Acetobacter aceti with an efficiency of conversion of about at best 85% of the stoichiometric theoretical. The overall weight yield from sugar to acetic acid with conventional technology is then about 49.5%.
A better process for making acetic acid is suggested by the knowledge of the existence of an organism known as Clostridium thermoaceticum which will convert sugars to acetic acid in one step. It is also attractive because it is a thermophile, that is, it grows best at temperatures around 60.degree. C. This reduces competition from contaminating organisms, so that maintaining a pure culture is easier. A large temperature differential from the cooling water is also provided, making heat removal easier. It is a strict anaerobe which is an asset in that the likelihood of contamination is reduced and a liability in that complete elimination of oxygen is necessary if the organism is to grow and function.
Unfortunately this organism only grows well at approximately neutral pH's. Since the product acetic acid reduces the pH, fermentation stops unless the acid is neutralized with a base such as sodium hydroxide. While this will then allow the organism to continue to ferment, it complicates the recovery of the desired product acetic acid because it must be removed as an acetic acid salt and then regenerated to the free acid.
It is therefore an object of this invention to provide a method utilizing Clostridium thermoaceticum to produce acetic acid by fermentation of carbohydrate without having to neutralize the acetic acid product.
State another way, it is another object of this invention to provide a form of Clostridium thermoaceticum which will grow in an aqueous nutrient at pH's below about 6.